The present invention relates to heat transfer element and, more specifically, to an assembly of heat absorbent plates for use in a heat exchanger wherein heat is transferred by means of the plates from a hot heat exchange fluid to a cold heat exchange fluid. More particularly, the present invention relates to an assembly of heat exchange element adapted for use in a heat transfer apparatus of the rotary regenerative type wherein the heat transfer element is heated by contact with the hot gaseous heat exchange fluid and thereafter brought in contact with a cool gaseous heat exchange fluid to which the heat transfer element gives up its heat.
One type of heat exchange apparatus to which the present invention has particular application is the well-known rotary regenerative heater. A typical rotary regenerative heater has a cylindrical rotor divided into compartments in which are disposed and supported spaced heat transfer plates which as the rotor turns are alternately exposed to a stream of heating gas and then upon further rotation of the rotor to a stream of cooler air or other gaseous fluid to be heated. As the heat transfer plates are exposed to the heating gas, they absorb heat therefrom and then when exposed to the cool air or other gaseous fluid to be heated, the heat absorbed from the heating gas by the heat transfer plates is transferred to the cooler gas. Most heat exchangers of this type have their heat transfer plates closely stacked in spaced relationship to provide a plurality of passageways between adjacent plates for flowing the heat exchange fluid therebetween.
In such a heat exchanger, the heat transfer capability of a heat exchanger of a given size is a function of the rate of heat transfer between the heat exchange fluid and the plate structure. However for commercial devices, the utility of a device is determined not alone by the coefficient of heat transfer obtained, but also by other factors such as the resistance to flow of the heat exchange fluid through the device, i.e., the pressure drop, the ease of cleaning the flow passages, the structural integrity of the heat transfer plates, as well as factors such as cost and weight of the plate structure. Ideally, the heat transfer plates will induce a highly turbulent flow through the passages therebetween in order to increase heat transfer from the heat exchange fluid to the plates while at the same time providing relatively low resistance to flow between the passages and also presenting a surface configuration which is readily cleanable.
To clean the heat transfer plates, it has been customary to provide soot blowers which deliver a blast of high pressure air or steam through the passages between the stacked heat transfer plates to dislodge any particulate deposits from the surface thereof and carry them away leaving a relatively clean surface. Many plate structures have been evolved in attempts to obtain cleanable structures with adequate heat transfer. See for example the following U.S. Pat. Nos.: 1,823,481; 2,023,965; 2,438,851; 2,983,486; and 3,463,222.
One problem encountered with this method of cleaning is that the force of the high pressure blowing medium on the relatively thin heat transfer plates can lead to cracking of the plates unless a certain amount of structural rigidity is designed into the stack assembly of heat transfer plates. One solution to this problem is presented in U.S. Pat. No. 2,596,642. As disclosed therein individual heat transfer plates are crimped at frequent intervals to provide double-lobed notches which have one lobe extending away from the plate in one direction and the other lobe extending away from the plate in the opposite direction. Then when the plates are stacked together to form the heat transfer element, these notches serve not only to maintain adjacent plates at their proper distance from each other, but also to provide support between adjacent plates so that forces placed on the plates during the soot blowing operation can be equilibrated between the various plates making up the heat transfer element assembly.
However, in a heat transfer element assembly comprised of a plurality of like notched plates in a stacked array, the potential exists for the notches of adjacent plates to nest. That is, the notches may all become superimposed on one another so that the spacing between adjacent plates is lost and the adjacent plates touch along their entire length or a significant portion thereof. This may occur from improper installation or movement of the plates relative to each other during normal operation or during the soot blowing procedure. In any case, this nesting should be avoided as fluid flow between adjacent plates is prevented when the plates become nested.
In U.S. Pat. No. 4,396,058, an assembly of heat transfer element for a rotary regenerative heat exchanger is provided wherein nesting of adjacent sheets is precluded. As disclosed therein, the heat transfer element assembly comprises a plurality of first and second heat absorbent plates stacked alternately in spaced relationship thereby providing a plurality of passageways between adjacent first and second plates for the flowing of a heat exchange fluid therebetween with spacers formed in the plate to extend between the plates to maintain a predetermined distance between adjacent plates. The spacers comprise bilobed folds in the first and second plates. To preclude nesting, the folds in the first plates have their first lobe projecting outwardly therefrom in a first direction and their second lobe projecting outwardly therefrom in a second direction which is opposite to the first direction, while the folds in the second plates have their first lobe projecting outwardly thereform in the second direction and their second lobe projecting outwardly therefrom in the first direction. Thus, the folds in the second plate have a pitch which is opposite to the pitch of the folds in the first plate. Because the folds of adjacent plates are opposite in pitch, there is no way that the folds of adjacent plates can become superimposed. Unfortunately, assembling such an array of heat transfer element is labor intensive and, therefore, such an array is significantly more expensive to manufacture than an array of like-notched sheets.
It is, therefore, an object of the present invention to provide an improved heat transfer element assembly wherein the structural integrity of the heat transfer plates is enhanced by crimping the plates with notches designed to preclude nesting, while at the same time providing a heat transfer element assembly the plates of which are relatively simple to manufacture and easy to assembly in a stacked array.